Electron beam apparatus

ABSTRACT

An electron beam ( 4 ) to be irradiated onto a sample ( 10 ) is two-dimensionally scanned by a scanning coil ( 9 ), and secondary electrons generated from the sample ( 10 ) by the scanning are detected by a secondary electron detector ( 13 ). A deflection coil ( 15 ) for image shifting is used for electrically deflecting the primary electron beam to shift a field of view for image shift in an arbitrary direction by an arbitrary amount. By the image shift, the primary electron beam ( 4 ) to be irradiated onto the sample is energy dispersed to degrade the resolution. However, an E×B field producer ( 30 ) for dispersion control gives the primary electron beam energy dispersion in the opposite direction and having the equal magnitude. Therefore, the energy dispersion produced in the primary electron beam by the image shift is automatically corrected.

This application is a continuation of application Ser. No. 10/111,459,filed Apr. 25, 2002, hereby incorporated by reference in its entirety.

TECHNICAL FIELD

The present invention relates to an electron beam apparatus, andparticularly to an electron beam apparatus such as a scanning electronmicroscope for performing evaluation of a micropattern in asemiconductor device or general-purpose observation of a general sample.

BACKGROUND ART

An electron beam apparatus such as a scanning electron microscopeobtaining a scanning image of a sample by scanning a primary electronbeam on the sample is used for a purpose of pattern evaluation such asinspection or measurement of a micropattern in a semiconductor device.The apparatus of such a kind comprises an image shift function forelectrically shifting an electron beam irradiation area (a field ofobservation view) within a range of several μm to 10 μm by electricallydeflecting the primary electron beam in order to shift the field ofobservation view to an evaluation point with high accuracy.

On the other hand, in a scanning electron microscope used for ageneral-purpose observation of a general sample, a field of observationview is mainly shifted by mechanically shifting a sample stage. However,because the mechanical shifting of the stage becomes difficult when themagnification of observation is high, image shift is used in order toshift a field of observation view to the observation center. In thiscase, since the field of observation view can be more speedily shiftedin a wide range, as the shifting amount of the field of observation viewby the image shift is larger, operability of the apparatus can beimproved.

Further, in the measurement of semiconductors or high-technologymaterials, a low acceleration voltage lower than several kV is generallyused in order to prevent samples from being charged, and necessity ofperforming nanometer order observation is increasing. Therefore, inorder to improve the resolution under the low acceleration voltage byreducing the aberration of the objective lens, the scanning electronmicroscope for this purpose is used by shortening the focal distance ofthe objective lens, or by applying a negative voltage to the sample(retarding method).

In the retarding method of applying a negative voltage to the sample,secondary electrons generated from the sample are accelerated by thevoltage applied to the sample to travel to the upper portion of theobjective lens. Therefore, as described in the specification of PatentNo. 2821153, by producing an electric field and a magnetic fieldintersecting each other at right angle (an E×B field) in the upperportion of the objective lens, the path of the secondary electronsgenerated from the sample is deflected to be separated from the path ofthe primary electrons traveling from an electron source, and thus thesecondary electrons are detected by a secondary electron detector inhigh efficiency. Further, the specification of Patent No.2821153discloses a method in which in order to eliminate the chromaticaberration produced by the E×B field, another E×B field is provided inthe side of the electron source at a position closer to the electronsource than the E×B field for deflecting the secondary electron, and thechromatic aberration of the E×B (E-cross-B) field for deflecting thesecondary electron is canceled by the chromatic aberration produced bythe E×B field provided in the side of the electron source. However, inthe technology described in the specification if Patent No.2B21153, theenergy dispersion to be corrected is only the energy dispersion producedin the deflection direction (only a single direction) of the secondaryelectrons.

The image shift function for shifting the field of observation view inan arbitrary direction by an arbitrary amount by electrically deflectingthe primary electron beam is a function indispensable to the electronbeam apparatus for obtaining a scanning image with high resolution andin high magnification, as described above. However, when image shift isperformed, the primary electron beam is energy-dispersed correspondingto the amount of image shift in the shift direction to cause degradationin the resolution. The degradation in the resolution becomes anun-negligible problem as the resolution of the apparatus is increased.

DISCLOSURE OF INVENTION

An object of the present invention is to provide an electron beamapparatus suitable for performing image shift in an arbitrary directionwithout degrading the resolution.

From a point of view, the present invention is characterized by anelectron beam apparatus comprising an electron source for emitting anelectron beam; a focusing lens for focusing the electron beam; and ameans for producing a scanned image of a sample by scanning the focusedelectron beam, which further comprises an energy dispersion controlmeans capable of energy-dispersing the electron beam and producing theenergy dispersion in an arbitrary direction on a plane intersecting anaxis of the electron beam.

From another point of view, the present invention is characterized by anelectron beam apparatus comprising an electron source for emitting anelectron beam; a focusing lens for focusing the electron beam; and ameans for producing a scanned image of a sample by scanning the focusedelectron beam, which further comprises means for generating an electricfield and a magnetic field intersecting each other at right angle on anaxis of the electron beam; and a means for controlling strengths of theelectric field and the magnetic field so that shift of a deflecteddirection of the electron beam by the electric field and the magneticfield is set to 180°, and a deflection angle ratio of the electron beamof the electric field to the magnetic field becomes 1 to 2.

The other objects and features of the present invention will appear inthe course of the following description referring to the accompaniedfigures.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing an embodiment of an electron beamapparatus in accordance with the present invention.

FIG. 2 is a diagram showing the construction of the dispersion controlE×B field generator of FIG. 1.

FIG. 3 is a view showing an example of a picture on the image display ofFIG. 1.

FIG. 4 is a flowchart showing an automatic adjustment flow of thedispersion control E×B field.

FIG. 5 is a graph showing the relationship between the dispersioncontrol E×B field and the amount of image shift.

FIG. 6 is a flowchart showing a procedure of determining correctioncoefficients and control coefficients in the dispersion control E×Bfield generator.

FIG. 7 is a view explaining path-dispersion caused by electricaldeflection of a primary electron beam.

FIG. 8 is a view showing cross-sectional shapes of a primary electronbeam before and after experienced energy dispersion.

FIG. 9 is a view explaining the principle of producing thepath-deflection of the primary electron beam by a focusing lens.

FIG. 10 is a diagram showing the construction of a general E×B fieldgenerator.

BEST MODE FOR CARRYING OUT THE INVENTION

An electron beam emitted from an electron source has deviation in energydue to characteristics of the electron source. For example, an electronbeam emitted from an electron source of a Schottky type obtainable of ahigh bright electron beam has an energy width of 0.4 eV to 0.6 eV. Whenthe electron beam having such energy deviation is electricallydeflected, the electron beam reaching a sample is energy dispersedcorresponding to the deviation in energy because the amount ofdeflection depends on the energy of the beam. Since the energydispersion causes a blur of the electron beam, the resolution isdegraded. Before describing concrete embodiments of the presentinvention, the principle of the degradation of resolution will bedescribed first.

Referring to FIG. 7, the primary electron beam 4 is deflected by amagnetic field produced by a deflection coil 15 to be irradiated onto asample 10 through a center of an objective lens 7. Letting a deflectedamount on the sample 10 of the primary electron beam deflected by thedeflection coil 15 be r_(d), the deflected amount r_(d) can be expressedas follows.R _(d) =k _(d) .I _(d) .V _(acc) ^(−1/2)  (1)Where, k_(d) is a deflection sensitivity coefficient determined by shapeand number of winding turns of the deflection coil 15, and I_(d) is acurrent of the deflection coil 15, and V_(acc) is an accelerationvoltage of the electrons passing through the deflection coil 15.

When the primary electrons of the acceleration voltage V_(acc) arereceived the deflecting action of r_(d) by the deflection coil 15, theelectron beam on the sample 10 receives path dispersion expressed by thefollowing equation by the energy deviation (energy dispersion) ΔV of theprimary electron beam 4 because change in the energy of the electronbeam is equivalent to change in the acceleration voltage. That is,Δr _(d)=(dr _(d) /dV _(acc)).ΔV=−0.5. r _(d).(ΔV/V _(acc))  (2)Since the path dispersion causes blur of the electron beam, theresolution is degraded. For example, providing that the accelerationvoltage (V_(acc)) is 1 kV, the image shift amount (r_(d)) is 10 μm, andthe energy width (ΔV) of the electron beam is 0.4 eV, path dispersion(blur of beam) calculated by Equation (2) becomes 2 nm. That is, whenimage shift of 10 μm is performed, the path dispersion (blur of beam) of2 nm occurs in a direction corresponding to the image shift. This meansthat, for example, in an apparatus capable of obtaining a resolution of10 μm, shape of the beam is changed from FIG. 8(a) to (b) as shown inthe figure by image shift of 10 μm, which means that the resolution isdegraded 4 nm in the direction of the path dispersion.

Although a magnetic field type objective lens is generally used forfocusing the primary electron beam, an electric field for deceleratingthe primary electron beam is often produced between the objective lensand a sample in order to further increase the focusing power. Thismethod is known as the retarding method. In such an electron opticalsystem, when the primary electron beam is deflected to be orthogonallyincident to the objective lens in order to shift the image, the pathdispersion is aggravated by the deceleration electric field between theobjective lens and the sample. The degree of the path dispersion by thedeceleration electric field between the objective lens and the sampledepends on the amount of image shift and the strength of thedeceleration electric field.

In order to focusing the primary electron beam onto the sample, aplurality of focusing lenses including the objective lens is generallyused. In such an electron optical system, when the primary electron beampasses along a path offsetting from the central axis of the focusinglens, in addition to the primary focusing action a deflecting actionalso appears in the focusing lens, as shown in FIG. 9. By thisdeflecting action, path dispersion caused by the energy deviation ΔV ofthe primary electron beam occurs similarly to the case of the imageshift.

Since a conventional electron beam apparatus cannot obtain theresolution equal to or lower than about 2 nm using an accelerationvoltage of 1 kV, the energy dispersion caused by the image shift or theoffset from the axis of the objective lens has not been recognized. Onlythe specification of Patent No.2821153 discloses a method of correctingan energy dispersing action in which the strong energy dispersing actionproduced in an E×B field provided for deflecting the path of acceleratedsecondary electrons is recognized to be only a factor degrading theresolution. Moreover, the correction method disclosed in thespecification of Patent No.2821153 is to correct only the dispersion ina single direction caused by the E×B field for deflecting the secondaryelectrons, but correction of the above-mentioned energy dispersiontwo-dimensionally produced in the arbitrary direction is not consideredat all. This comes from that the effect of the energy dispersion causedby the image shift and the offset from the axis of the objective lenscannot have been recognized.

The energy dispersion of the primary electron beam can be corrected byproducing opposite-direction energy dispersion. However, in the casewhere the dispersing action occurs two-dimensionally in an arbitrarydirection as described above, it is necessary that energy dispersion forcorrecting the above energy dispersion is produced in an arbitrarydirection.

In order to produce the energy dispersion in the primary electron beam,an electric deflecting action should be given to the primary electronbeam. At that time, when the deflection action is produced by a magneticfield and an electric field in directions opposite to each other, onlyenergy dispersion caused by deviation of the beam energy can be producedin the primary electron beam without deflecting the path to the primaryelectrons of the average energy. In order to do so, the electric fieldand the magnetic field giving the deflecting action to the primaryelectron beam should be produced so as to intersect each other at rightangle. This principle is known as the Wein filter or the E×B (E crossB).

The E×B is usually used for improving uniformity in energy of theprimary electron beam (an energy filter) or for separating electronshaving largely different energy or electrons traveling in directionsopposite to each other. In this purpose, an E×B composed of a pair ofelectrodes and a pair of magnetic field generators shown in FIG. 10 isgenerally used because it is sufficient to produce the deflecting actiononly in a specified direction. Otherwise, as described in JapanesePatent Application Laid-Open No.6132002, a pair of electrodes and twopairs of coils are arranged to improve the orthogonality between theelectric field and the magnetic field.

On the other hand, the energy dispersion of the primary electronsgenerated by the deflection such as image shift will be produced all thedirections corresponding to the moving direction of image shift.Therefore, it is necessary to control the direction and the magnitude ofthe electric field and the magnetic field corresponding to the motion ofthe image shift. Accordingly, the means for producing the E×B field fordispersion control is composed of two pairs of electric field generatorsand two pairs of magnetic field generators arranged orthogonally to eachother, as shown in FIG. 2.

Description will be made below on the method of producing the energydispersion of an arbitrary direction and an arbitrary magnitude in theprimary electron beam on the plane crossing (intersecting) the axis ofthe electron beam at right angle, referring to FIG. 2.

Referring to FIG. 2, the electric field generators and the magneticgenerators are arranged so that the reflection fulcrums to the primaryelectron beam are positioned on a single plane. The electric fieldgenerators are composed of a first pair of E×B control electrodes 36 a,36 b and a second pair of E×B control electrodes 37 a, 37 b producingelectric fields intersecting each other at right angle, and the firstpair of electrodes and the second pair of electrodes are individuallyarranged opposite to each other. The magnetic field generators arecomposed of a first pair of E×B control deflection coils 38 a, 38 b anda second pair of E×B control deflection coils 39 a, 39 b producingmagnetic fields intersecting each other at right angle, and the firstpair of coils and the second pair of coils are individually arrangedopposite to each other. The electric field produced by the first pair ofelectrodes intersects the magnetic field produced by the first pair ofcoils at right angle, and the electric field produced by the second pairof electrodes intersects the magnetic field produced by the second pairof coils at right angle.

In regard to the E×B control voltages Vx, Vy, voltages having differentpolarities (Vx, −Vx), (Vy, −Vy) are applied to the individual pairs ofelectrodes (36 a, 36 b, 37 a, 37 b), respectively. On the other hand,the control voltages (Vx, Vy) are also supplied to a voltage synthesiscircuit 35 to be performed with voltage-current conversion expressed bythe following equations, and then supplied to the individual pairs ofE×B control coils (38 a, 38 b, 39 a, 39 b) as currents Ix, Iy.Ix=(Kxx.Vx+Kxy.Vy).Vacc ^(−1/2)  (3)Iy=(Kyx.Vx+Kyy.Vy).Vacc ^(−1/2)  (4)

There, the values Kxx, Kxy, Kyx and Kyy are set so that the deflectingactions of the primary electron beam by the electric fields produced bythe voltage Vx and Vy and by the magnetic fields controlled by Equation(3) and Equation (4) are canceled each other, respectively. This settingcan be performed, for example, by modulating or ON/OFF operating thecontrol voltage Vx to monitor motion of the primary electron beam on thesample (motion of the scanned image), and adjusting the condition of Kxxand Kxy so as to stop the motion. Otherwise, the E×B control voltage Vxis set to preset different conditions (for example, 0V and 5V), and ameans for capturing images obtained from each of the conditions isprovided to detect displacement between the captured images, throughimage processing, and then the optimum condition of Kxx and Kxy can beautomatically determined from the displacement through calculationpredetermined by an experiment, a simulation or the like. In regard tothe voltage Vy, the values Kyx and Kyy can be set the similaradjustment. Once these values are adjusted, the condition of alwayscanceling the deflecting action of the primary electron beam by the E×Bfield can be held by the acceleration voltage linked control (Vacc⁻1/2)in Equation (3) and Equation (4) even if the acceleration voltage Vaccis varied. Therefore, by controlling the E×B control voltages Vx and Vy,energy dispersion corresponding to the control voltages can be producedin the primary electron beam without giving any deflecting action to theprimary electron beam.

As described above, according to the E×B field generator of FIG. 2, thecondition of canceling the deflecting action to the primary electronbeam (the Wien condition) is satisfied, and at the same time the energydispersion can be produced in an arbitrary direction. Therefore, bypre-understanding the direction and the magnitude of the energydispersion received by the primary electron beam focused onto a sampleto the direction and the magnitude of image shift, control of the E×B inlinking with the image shift can be performed.

In the case where an electric field for decelerating the primaryelectron beam is produced between the objective lens and the sample toincrease the resolution, the magnitude of energy dispersion produced inthe primary electron beam also depends on the decoration electric field.Therefore, the control is performed in linking with not only the imageshift but also the strength of the deceleration electric field (forexample, in linking with the negative voltage applied to the sample).This control can be performed by, pre-investigating the relationshipbetween the strength of the deceleration electric field and thecorrection amount of the energy dispersion through calculation or anexperiment, and then installing the relationship into the controlprogram.

Further, in regard to energy dispersion caused by a factor other thanthe image shift such as energy dispersion caused by the primary electronbeam passing through out of the axis of the focusing lens, it ispossible to apply E×B control voltages so as to producing energydispersion canceling the energy dispersion caused by the factor otherthan the image shift. However, since the energy dispersion componentcaused by the offset of the axis of the focusing lens depends on theaccuracy of axis adjustment, a means capable of manually adjusting(correcting) the energy dispersion is provided.

Further, the energy dispersion received by the primary electron beam tothe intensities of the electric field and the magnetic field composingthe E×B varies depending on not only the acceleration voltage but alsodepending on the operating condition of the electron optical systemtypical of the focusing distance of the lens. Further, in theconstruction of electron optical system in which an accelerationelectrode for accelerating the primary electrons is arranged theobjective lens portion in order to improve the resolution of lowacceleration voltage, the magnitude and the direction produced in energydispersion are varied by the voltage applied to the accelerationelectrode. Therefore, it is possible to avoid the energy dispersion ofthe primary electron beam (degradation of the resolution) to thearbitrary operating condition of the optical system or the arbitraryimage shift by pre-understanding the relationship between the controlvoltage of the E×B and the operating parameters of the electron opticalsystem or the applied voltage of the acceleration electrode of theobjective lens through an experiment or simulation to link the controlvoltage of the E×B the operating parameters of the electron opticalsystem.

In the case where the energy dispersion of the primary electron beam ismainly caused by the image shift, a beam deflection means not producingenergy dispersion can be employed. It has been found a method that byemploying a means (an E×B field producing means) for producing anelectric field and a magnetic field in an arbitrary direction whilemaintaining to intersect each other at right angle as the means of thistype, the ratio of the deflection angle by the magnetic field of theprimary electron beam to the deflection angle by the electric field ismaintained be two times. According to this method, energy dispersionnever produced to an arbitrary beam deflection at all from the followingreason.

Deflection angles θE and θB of electrons of energy Vacc by the electricfield E and the magnetic field B can be expressed as follows,respectively.θE=KE.VE/Vacc  (5)θB=KB.IB/√{square root over (Vacc)}  (6)Here, letting the variations (dispersions) of deflection angle of theelectron beam by the energy deviation ΔV be ΔθE and ΔθB, respectively,the following equations can be obtained. That is,ΔθE=(dθE/dVacc).ΔV=−(ΔV/Vacc).θE  (7)ΔθB=(dθB/dVacc).Δv=−0.5(ΔV/Vacc).θB  (8)When the deflection amounts by the electric field and the magnetic fieldare equal to each other, the magnitude of energy dispersion produced bythe electric field is two times as large as that produced by themagnetic field. Therefore, if the intensities of the electric field andthe magnetic field of the E×B field are set so that the deflection angleby the magnetic field becomes two times as large as the deflection angleby the electric field and that the deflection directions of the primaryelectron beam by the electric field and the magnetic field becomesopposite to each other, the energy dispersions by the electric field andthe magnetic field are canceled by each other. In other words, lettingthe amount of deflection of the primary electron beam by the electricfield (the value on the sample) be rd, and the amount of deflection ofthe primary electron beam by the magnetic field be 2rd in the directionopposite to the direction of the deflection by the electric field,energy dispersion associated with the beam deflection is not produced atall, and accordingly the degradation of the resolution does not occur.At that time, the total amount of deflection of the primary electronbeam becomes the difference rd between the amounts deflected by themagnetic field and the electric field (because the deflected directionscaused by the magnetic field and the electric field are opposite to eachother), and accordingly the deflecting means not producing energydispersion can be materialized. In an image shift function in a commonapparatus, when the primary electron beam is deflected using the beamdeflecting means arranged above the objective lens, two deflecting meansare arranged in two stages along the optical axis and operated so thatthe aberration of the objective lens produced by the deflection may beminimized. By replacing these two-stage beam deflecting means by theabove-mentioned E×B fields each controlled so that the amounts ofdeflection of the electric field and the magnetic field may becometwice, an image shift function not producing the degradation of theresolution caused by the energy dispersion can be realized.

Further, the E×B field controlled so that the amounts of deflection ofthe electric field and the magnetic field may become twice may be usednot only for image shift as a deflector not producing chromaticaberration but also for a deflector for aligning the primary electronbeam.

A preferred embodiment in accordance with the present invention will bedescribed below. Referring to FIG. 1, a voltage is applied between acathode 1 and a first anode 2 by a high voltage control power source 20controlled by a microprocessor (CPU) 40 to extract a primary electronbeam 4 in a preset emission current from the cathode 1. Since anaccelerating voltage is applied between the cathode 1 and a second anode3 by the high voltage control power source 20 controlled by the CPU 40,the primary electron beam 4 emitted from the cathode 1 is acceleratedand travels to a post lens system. The primary electron beam 4 isfocused by a focusing lens 5 controlled by a lens control power source21, and after removing an unnecessary region of the primary electronbeam by an aperture plate 8, the primary electron beam 4 is focused on asample 10 as a micro-spot by a focusing lens 6 controlled by a lenscontrol power source 22 and an objective lens 7 controlled by anobjective lens control power source 23. The objective lens 7 is a kindof focusing lens, and each of the focusing lens 6 and the objective lens7 is of a magnetic field type in which a focusing lens action isgenerally produced by conducting exciting current.

The primary electron beam 4 is two-dimensionally scanned on the sample10 by a scanning coil 9. A secondary signal (a sample signal) 12 such assecondary electrons generated from the sample by irradiating the primaryelectron beam travels above the objective lens 7, and then is separatedfrom a path of the primary electron beam 4 and deflected toward asecondary signal detector 13 by an orthogonal electric-magnetic fieldgenerator 11 for separating the secondary signal. The reflectedsecondary signal 12 is detected by the secondary signal detector 13. Asignal of the secondary signal detector 13 is stored in an image memory25 as an image signal through a signal amplifier 14. The imageinformation stored in the image memory 25 is displayed on an imagedisplay unit 26 at any time. A signal to the scanning coil 9 iscontrolled by a scanning coil control power source 24 corresponding toan observation magnification ratio. A deflecting coil 15 for image shiftis arranged at a position of the scanning coil 9, and is controlled byan image shift control power source 27 corresponding to a necessaryshifting amount of field of view.

A negative voltage VR (retarding voltage) is applied to the sample 10 todecelerate the primary electrons at a position just before the sample.Further, a booster electrode 16 for temporarily accelerating the primaryelectrons is arranged in a portion of the objective lens 7, and apositive voltage is applied to the electrode. The retardant voltage andthe booster voltage are controlled by the control CPU 40 to improve theresolution under a low acceleration voltage condition.

A dispersion control E×B field generator 30 capable of giving arbitraryenergy dispersion to the focused electron beam on the sample 10 isarranged between the scanning coil and the focusing lens 6, and iscontrolled by a control power source 31. The dispersion control E×Bfield generator 30 and the control power source 31 have the constructionshown in FIG. 2, and the control voltages Vx and Vy and the coilcurrents Ix and Iy are controlled by the control CPU 40 so as to satisfyEquation (3) and Equation (4).

A knob 32 on an operating panel is connected to the control CPU 40, andvalues of the coefficients (Kxx, Kxy, Kyx, Kyy) of Equation (3) andEquation (4) can be input to the control CPU 40 by adjusting the knob 32at adjusting the dispersion control E×B. At adjusting the dispersioncontrol E×B, signals are given from the control CPU 40 to the controlpower source 31 so that the control voltages (Vx, Vy) of the dispersioncontrol E×B may be modulated one after the other. Therefore, an operator(an adjuster) can set the optimum values of the coefficients (Kxx, Kxy,Kyx, Kyy) of the dispersion control E×B by adjusting the knob 32 so asto eliminate movement of an image associated with the modulation of thecontrol voltages (Vx, Vy). The adjusting of the coefficients of thedispersion control E×B can be performed by controlling a curser or ascroll bar displayed on a monitor CRT using a mouse pointer instead ofusing the knob 32.

FIG. 3 is a view showing an example of a picture (an adjustment picture)for adjusting of the coefficients of the dispersion control E×B on themonitor CRT. When the coefficients of the dispersion control E×B areadjusted, the picture of FIG. 3 is displayed, and then the coefficientsare set according to the following procedure.

-   (1) A Wobbler X button in the adjustment picture is clicked using    the mouse. At that time, the control CPU 40 controls the control    power source 31 so that the control voltage Vx periodically varies.-   (2) Since the field of view of the scanned image is moved    corresponding to the variation of the control voltage Vx when the    values of the coefficients Kxx and Kxy of the dispersion correction    E×B are improper, slide bars XX and YY in the adjustment picture are    adjusted so that the variation of the field of view is substantially    eliminated. Numerical values displayed in the right side of the    slide bars express the set coefficients, and numerical values may be    directly input into the displayed windows.-   (3) A Wobbler Y button in the adjustment picture is clicked using    the mouse. By this operation, the control voltage Vy is periodically    varied-   (4) Slide bars YX and YY in the adjustment picture are adjusted so    as to stop movement of the scanned image.-   (5) The adjusted coefficients (KXX, Kxy, Kyx, Kyy) are stored in the    memory of the control CPU by clicking the Save button on the    adjustment picture.-   (6) After completion of the adjustment, the adjustment picture is    closed by clicking the Exit button using the mouse.

If an image processing function for detecting the movement of the imagecaused by the modulation of the control voltages (Vx, Vy) is provided,it is possible to automatically set the coefficients (Kxx, Kxy, Kyx,Kyy) so as to eliminate the movement of the image.

FIG. 4 is a flowchart showing a processing flow of automaticallyexecuting the above-mentioned adjustment operation by the imageprocessing. In the case of determining the coefficients Kxx and Kxy inregard to the control voltage Vx, images for cases where Vx is 0 V andVx is +5V are captured (S1, S2). The correlation between the bothcaptured images is calculated while the both captured images arerelatively being shifted in the X- and Y-directions pixel by pixel (S3).A shifting amount between the both images making the correlation maximumis searched (S4). The correlation between the both images becomes themaximum when the fields of view of the both images agree and overlapwith each other. From the shifting amount of the images when thecorrelation between the images becomes the maximum, a displacement (amagnitude and a direction) of the field of view to the variation ofcontrol voltage Vx (5V) is accurately calculated (S5). Similarly, bycapturing images with respect to different values of the coefficientsKxx and Kxy, a displacement amount of the image by each of thecoefficients can be quantitatively and automatically calculated (S6,S7). From the result, the coefficients Kxx and Kyy are calculated andset so as to cancel out the displacement of the image by the controlvoltage Vx (S8). Setting of the coefficients Kyx and Kyy with respect tothe control voltage Vy is similarly executed.

FIG. 5 is a graph showing an example expressing the relationship betweenthe control voltages of the dispersion control E×B and the amount ofimage shift when the image shit is performed in the X-direction. Whenthe image shift is 0, offset values of the control voltages (Vx, Vy) ofthe dispersion control E×B are set so that the path dispersion caused byfactors other than the image shift are canceled. The offset valuesinclude corrections of the path dispersion caused by the offset of theaxis of the focusing lenses other than the objective lens and of thepath dispersion produced in the E×B field for deflecting the secondaryelectrons. Since the direction of image shift (the X-direction in theexample of FIG. 3) does not agree with the direction of the pathdispersion or since the direction of the image shift coil (X, Y) doesnot always agree with the electric field (X, Y) for controlling thedispersion Ayx and Ayy, the both of the voltages Vx and Vy forcontrolling the dispersion are controlled in linking with each other tothe X-direction of image shift. Further, the relationship between thecontrol voltage Vx and Vy and the image shift is controlled under apreset condition in linking with the acceleration voltage and theintensity of the sample and the objective lens (in FIG. 1, the retardingvoltage VR and the voltage VB of the booster electrode 16 of theobjective lens portion).

In FIG. 5, the values of the control voltages Vx and Vy under thecondition of the zero-image shift are controlled in linking with thevariation of the focusing point of the primary electron beam by thefocusing lens 6.

The correction of energy dispersion produced by the image shift or theother factors is performed by controlling the control parameters Vx andVy in the E×B field generator 30 as follows.Vx=Axx.Xi+Axy.Yi+Vx _(o)  (9)Vy=Ayx.Xi+Ayy.Yi+Vy _(o)  (10)

There, Xi and Yi express amounts of control of the image shift (amountsof deflection of the electron beam on the sample). The controlcoefficients Axx, Vx₀, Ayx, Ayy, Vy_(o) are calculated in advance foreach different optical condition such as the acceleration voltage andthe deceleration electric field through the procedure of FIG. 6, andstored in an external memory unit 41.

As the optical condition of the apparatus such as the accelerationvoltage, the decoration electric field and so on is set, the CPU 40reads out the corresponding control coefficients (Axx, Axy, Vx_(o),Vy_(o)) from the external memory unit 41 to execute control in linkingwith the control of the image shift so that the relationship of Equation(9) and Equation (10) are satisfied.

FIG. 6 shows a procedure of determining the correction coefficients andthe control coefficients. The correction coefficients Kxx, Kxy, Kyx andKyy of Equation (3) and Equation (4) are determined using the adjustmentmeans (the knob 32) of the dispersion correction means (the dispersioncontrol E×B field generator 30) (S1), and the path dispersion of theelectron beam is corrected under a condition of setting both of theimage shift control amounts Xi and Yi to 0, and then the controlparameters Vx and Vy of the dispersion correction means capable ofcompletely correcting the dispersion are defined as the controlcoefficients Vx_(o), and Vy_(o), respectively (S2). Further, by settingthe image shift amount Xi to a preset value, the control parameters Vxand Vy of the dispersion correction means capable of completelycorrecting the path dispersion of the electron beam are adjusted, andthe control coefficients Axx and Axy of the dispersion correction meansare determined from the condition (S3). Further, by setting the imageshift amount Yi to a preset value, the control parameters Vx and Vy ofthe dispersion correction means capable of completely correcting thepath dispersion of the electron beam are adjusted, and the controlcoefficients Ayx and Ayy of the dispersion correction means aredetermined from the condition (S4). The correction coefficients and thecontrol coefficients determined through the above-mentioned course arestored in the external memory unit 41.

According to the present embodiment described above, the path dispersioncaused by the energy dispersion of the primary electrons produced by theimage shift or the other factors can be canceled. Therefore, in anelectron beam apparatus capable obtaining high resolution particularlyunder low acceleration voltage, it is possible to prevent the resolutionfrom being degraded, and to perform the image shift in an arbitrarydirection.

As it is obvious that many changes and modifications may be made withoutdeparting from the essence of the present invention, it is to beunderstood that the present invention is not limited to theabove-described embodiment.

1. An electron beam apparatus comprising an objective lens for focusingan electron beam emitted from an electron source, further comprising: adeflector for deflecting said electron beam from an optical axis of saidobjective lens; a Wien filter for dispersing said electron beam so thatthe aberration of said electron beam, which appears when said electronbeam is deflected from said optical axis of said objective lens by saiddeflector, becomes lessened; and a control device for setting a workingcondition of said Wien filter so that an image displacement, which iscaused by irradiation of said electron beam when said working conditionof said Wien filter is varied, is reduced.
 2. An electron beam apparatusaccording to claim 1, wherein said Wein filter comprises an electricfield generator having two pairs of electrodes and a magnetic fieldgenerator.
 3. An electron beam apparatus according to claim 2, whereinsaid control device impresses a periodically altering control voltage oneach of said two pairs of electrodes to vary said working condition ofsaid Wien filter.
 4. An electron beam apparatus according to claim 2,wherein said control device imports images for every impression ofdifferent control voltages on said electric field generator to correlatesaid imported images, shifting one image relatively from another image,and computes an amount of positional shift between said imported imagesin terms of said altered control voltages based on a shifting degreethat shows a close correlation.
 5. An electron beam apparatus accordingto claim 4, wherein said electric field generator has two pairs ofelectrodes; and said control device controls voltages acrosspair-electrodes in said two pairs of electrodes to eliminatedisplacement of an image caused from variation in the voltage acrosssaid pair-electrodes in either of said two pairs of electrodes.
 6. Anelectron beam apparatus comprising an objective lens for focusing anelectron beam emitted from an electron source, further comprising: adeflector for deflecting said electron beam from an optical axis of saidobjective lens; a Wien filter for dispersing said electron beam so thatan aberration of said electron beam, which appears when said electronbeam is deflected from said optical axis of said objective lens by saiddeflector, becomes lessened; and a control device for varying adispersing condition of said Wien filter according to an accelerationvoltage of said electron beam.
 7. An electron beam apparatus accordingto claim 6, wherein said control device forms a decelerating electricfield between said objective lens and a specimen to which said electronbeam is irradiated and varies said dispersing condition of said Wienfilter according to a variation in said decelerating electric field. 8.An electron beam apparatus comprising an objective lens for focusing anelectron beam emitted from an electron source and an accelerationelectrode for accelerating said electron beam, further comprising: adeflector for deflecting said electron beam from an optical axis of saidobjective lens; a Wien filter for dispersing said electron beam so thatan aberration of said electron beam, which appears when said electronbeam is deflected from said optical axis of said objective lens by saiddeflector, becomes lessened; and a control device for varying adispersing condition of said Wien filter according to a voltageimpressed on said acceleration electrode.